In recent years, flat displaying device (hereinafter, referred to as a flat display) are used in many fields and in many places, and their importance is being increased as the information age progress.
At the present day, a liquid crystal display (hereinafter, referred to as LCD) is a typical flat display. However, as flat displays based on a displaying principle that is different from that of the LCD, organic EL, inorganic EL, plasma display panels (hereinafter, referred to as PDP), light emitting diode displays (hereinafter, referred to as LED), vacuum fluorescent displays (hereinafter, referred to as VFD), field emission displays (hereinafter, referred to as FED) and the like are developed actively.
These new flat displays are called spontaneous light emitting type displays, and they differ greatly from LCD in the following points and have excellent characteristics which are not provided to LCD.
The LCD is called a light receiving type display. The liquid crystal does not emit light and operates as a so called shutter which transmits or shields external light so as to configure a display.
For this reason, the LCD requires light sources, in general, requires back light.
On the contrary, the spontaneous light emitting type displays do not require individual light sources because the displays themselves emit light.
In the light receiving displays such as LCD, back light is always on regardless of a mode of display information. Thus, these displays consume electric power which is approximately the same as the electric power in a full displaying state.
On the contrary, in the spontaneous light emitting type displays, since only a part which should be on according to display information consumes electric power, these devices have an advantage such that the power consumption is less than the light receiving type displays in principle.
In LCD, since light of the back light source is shield so that a dark state is obtained, it is difficult to eliminate light leakage completely, even in a small amount. On the contrary, in the spontaneous light emitting type displays, since a non-emitting state is just a dark state, an optimal dark state can be easily obtained, and the spontaneous light emitting type displays have an overwhelming advantage in contrast.
Moreover, since the LCD utilizes polarization control due to double refraction of the liquid crystal, its so called view angle dependence, in which a displaying state greatly changes according to an observation direction, is strong. The spontaneous light emitting type displays, however, scarcely have such a problem.
Since the LCD utilizes an alignment change, originated from dielectric anisotropy of the liquid crystal as an organic elastic substance, response time to an electric signal is 1 ms or more in principle.
On the contrary, since the above technique, which is being developed, utilizes carrier transition, electron emission, plasma discharge and the like such as electron/hole, the response time has ns digits. The response speed is incomparably higher than the liquid crystal, and a problem of afterimage of an animation caused by low speed response in LCD does not occur.
Among the above, the study of organic EL is particularly active.
The organic EL is also called OEL (Organic EL) or organic light emitting diode (OLED).
An OEL element and an OELD element are configured so that an EL layer comprising an organic compound is sandwiched in between a pair of electrode, an anode and a cathode. This configuration is based on a laminated configuration of “anode electrode/hole injection layer/light emitting layer/cathode electrode” of Tang et al. (Japanese Patent No. 1526026).
Moreover, in contrast to Tang et al. using a low molecular weight material, Nakano et al. use a high molecular weight material (Japanese Patent Application Laid-Open No. 3-273087).
Further, efficiency is improved by using the hole injection layer or an electron injection layer, or a light emitting layer is doped with fluorescent dye or the like so that a luminescent color is controlled.
Here, a pixel electrode and a facing electrode correspond to either of the anode and the cathode, so that a pair of electrodes is formed.
All the layers provided in between the pair of electrodes are generally called EL layers, and above mentioned hole injection layer, a hole transportation layer, a light emitting layer, an electron transportation layer and an electron injection layer are included.
FIG. 12 is a diagram showing a sectional configuration of an organic EL element.
The organic EL emits light by applying an electric field between the electrodes so as to apply the current to the EL layer. Conventionally, only fluorescence, light emission when returning to the ground state from the singlet excited state, is used. However, phosphorescence, light emission when returning to the ground state form the triplet excited state, can be effectively utilized due to recent studies. And thus, the efficiency is improved.
Normally, one electrode is formed on a light transmitting supporting substrate 24 such as a glass substrate or a plastic substrate, and an EL layer (light emitting layer) 26 and a facing electrode are formed in this order.
The electrode to be formed on the substrate may be an anode 25 or a cathode 27, and as a result, there are a bottom emission configuration shown in FIG. 12, wherein light is emitted 28 to the substrate side, and a top emission configuration shown in FIG. 13, wherein light is emitted 28 to an opposite direction to the substrate.
In the case of the top emission configuration, light transmittance is not required to the substrate.
A study is being conducted in which light taking-out efficiency is improved by taking out the light emission deactivated due to optical waveguide effect of the light transmitting substrate, by using a low refractive index material.
Though not shown in FIGS. 12 and 13, since the organic EL element is remarkably deteriorated due to water and oxygen, an inactive gas is generally filled so that the element does not come into contact with water or oxygen, and then, another substrate is used or so called sealing is carried out by thin film evaporation, so that reliability is secured.
As a method for forming an EL layer, a vacuum evaporation method is generally used for a low molecular weight material, and spin coating, a printing method or a transfer method is used for a high molecular weight material, in a form of a solution.
When a color display is manufactured by forming fine pixels with different light emitting color materials, a mask evaporation method is used for a low molecular weight material, and an ink-jet method, the printing method, the transfer method or the like is used for a high molecular weight material.
When the organic EL element is utilized as a display, similarly to LCD, its system can be roughly divided into a passive matrix mode and an active matrix mode, according to an electrode configuration and to a driving method.
The passive matrix mode has a simple configuration such that a pair of electrodes is formed with a horizontal electrode and a vertical electrode, which crosses each other with an EL layer in between. In order to display an image, however, instantaneous brightness should be heightened by a multiple of a number of scanning lines due to time-divisional scanning. The instantaneous brightness of the organic EL over 10000 cd/m2 is required in displays of normal VGA or more. And thus, a lot of practical problems as a display will occur.
In the active matrix mode, a pixel electrode is formed on a substrate formed with TFT, and an EL layer and a facing electrode are formed. Its configuration is more complicated than that of the passive matrix mode, but this mode has a lot of advantages as the organic EL display in light emitting brightness, power consumption, and crosstalk.
Further, since the active matrix mode displays using polysilicon films has higher electric field effect mobility than that of amorphous silicon films, a high current process on TFT is possible, and this mode is suitable for driving the organic EL as an electric current driving element.
Since a high speed operation is possible in a polysilicon TFT, various control circuits, which are conventionally processed by external ICs, are formed on the same substrate as displaying pixels, and the display has a lot of advantages such as miniaturization of the display, reduction of the cost, multifunction and the like.
As mentioned above, the organic EL display has a lot of properties. However, it has practical disadvantages in comparison with the light receiving type LCD.
In the LCD, with the back light being on, the light transmission and non-light transmission of each pixel is switched by a shutter effect, and thus, state of brightness and darknesss is controlled so that an image is displayed. For this reason, even if a background is in a bright state and the image information is in a dark state, namely, normally white display, or even if the background is in the dark state and the image information is in the bright state, namely, a normally black display, the power consumption is not changed, and either can be selected according to applications or user's preference.
On the contrary, in the conventional organic EL displays, since the bright state is created by light emission, the so called normally black display, in which the background not emitting light is in the dark state and the image information is in the bright state, cannot be avoided.
Regardless of such a technical aspect, the normally white display is preferable for text display because of the very long cultural history such that characters are written on white paper using writing instruments.
In the conventional organic EL displays of the normally black display, display is as shown in FIG. 7, and thus this display is not suitable for user's preference.
As mentioned above, the current organic EL displays are characterized by contrast and high animating performance, and they are suitable for so called graphic display such as graphic user interface (GUI) images and animation display. These displays are not, however, suitable for text display which requires the normally white display.
In the conventional displays, information to be displayed differs according to instruments to which displays are installed, such as TV for displaying animations and PC for displaying still images and text.
However, when all instruments such as TV, PC, digital camera, and PDA are connected to a network due to the development of the internet and the improvement of a communicating speed, a function for displaying information, of which animations, still images, texts and the like are combined, is required to displays.
For example, while an animation is being displayed on a part of the display, a text is displayed on another part. Like such a case, it is required that various kinds of information are displayed on one display.
In order to provide the normally white display, which is popular among users, with the spontaneous light emitting type organic EL displays, it is necessary to turn on all the pixels, and then, turn off the pixels on which image information are present. This method cannot take enough advantage of less power consumption, at the time, when only a portion which should be on is turned on according to the display information.
Conventionally, there is no display which can provide normally white display efficiently while taking advantage of the excellent performance, of the spontaneous light emitting type organic EL displays, at the same time.
FIG. 15 is a diagram showing a signal processing system of a conventional active matrix mode organic EL display.
A gate driver 12 and a data driver 13 are operated according to a scanning signal and a data signal controlled by a controller 11, respectively. Thus, ON/OFF state of the pixels is controlled.
A power source circuit 14 is for supplying an electric current to an organic EL element which is one kind of light emitting diode. Electric current is supplied to pixels which are controlled to be on, and the organic EL element emits light.
In such an active matrix mode organic EL device, in addition to power consumption of the element due to the pixel light emission, power consumption due to the driver operation of the gate driver 12 and the data driver 13 is important as the entire display system.
That is, even if the light emitting area, which is determined by a total number of light emitting pixels, is the same, when the power consumption of the drivers is small, the entire display consumes low power, being an efficient display.
The power consumption of the drivers is determined by an operating frequency, namely, a number of times of rewriting a signal, and thus, as the operating frequency is low and the frequency of rewriting a signal is fewer, the power consumption can be lower.
FIG. 11 is a typical pixel circuit configuration of a conventional organic EL display.
The circuit is configured by a switching TFT (4), a gate retention capacity (5), a driving TFT (6), and an EL element (7), in addition to each bus line of a scanning line G (1), a data signal line D (2), and a power supply line V (3).
When a gate of the switching TFT (4) selected by the scanning line G (1) is opened and a signal voltage according to light emitting intensity is applied to a TFT source from the data signal line D (2), a gate of the driving TFT (6) is opened according to a level of the signal voltage in an analog manner. This state is retained at the gate retention capacity (5).
When the voltage is applied to a source of the driving TFT (6) from the power supply line V (3), an electric current according to an opening level of the gate flows through the EL element (7), and the EL element (7) emits light according to the signal voltage in gradation.
In a display having such a circuit configuration, at the normal time, when the switching TFT (4) is not selected, the organic EL element (7) is in the non-light emitting state, and when the switching TFT (4) is selected, the organic EL element (7) will be in the light emitting state and will be in normally black display shown in FIG. 7 is provided.
Besides the above, the circuit configuration and the driving method of the organic EL display includes digital gradation driving methods such as a method wherein a number of TFTs is further increased (“Pixel Driving Methods for Large-Sized Poly-si AM-OLED Displays” by Yumoto et al., Asia Display/IDW′ 01 P. 1395–1398), time divisional gradation (“6-bit Digital VGA OLED” by Mizukami et al., SID′ 00 P. 912–915), and area divisional gradation (“Full Color Displays Fabricated by Ink-Jet Printing” by Miyasita et al., Asia Display/IDW′ 01 P. 1399–1402). They provide the normally black display as well.
In order to provide the normally white display with the organic EL display, as shown in FIG. 8, all the gate scanning lines are selectively scanned, while selecting the data lines corresponding to the most of pixels with no image information and making the background in the light emitting state. At the same time, only data lines of pixels with image information need to be non-selected state and be in the non-light emitting state.
Since the background in the bright state, namely, selected pixels are inevitably present on all the scanning lines and the data lines, as scanning conditions, both the gate lines and data lines will be operated by entire screen scanning and entire screen selection.
Since the entire screen scanning and the entire screen selection are carried out, the power consumption for operating the drivers, for the gate driver and the data driver added together, becomes maximum.